Methane Conversion to Aromatics on Mo/H-ZSM5: Structure of Molybdenum Species in Working Catalysts

The structure and density of Mo species in Mo/H−ZSM5 during catalytic CH 4 reactions was investigated using in-situ X-ray absorption spectroscopy (XAS), temperature-programmed oxidation after reaction, and the isotopic exchange of D 2 with OH groups in H−ZSM5 before and after CH 4 reactions. These methods reveal that CH 4 reactions cause exchanged Mo 2 O 5 2+ dimers, formed from physical mixtures of MoO 3 and H−ZSM5, to reduce and carburize to form small (0.6−1 nm) MoC x clusters with the concurrent regeneration of the bridging OH groups that were initially replaced by Mo oxo dimers during exchange. In this manner, catalytically inactive Mo oxo species activate in contact with CH 4 to form the two sites required for the conversion of CH 4 to aromatics: MoC x for C−H bond activation and initial C−C bond formation and acid sites for oligomerization and cyclization of C 2+ hydrocarbons to form stable aromatics. These MoC x clusters resist agglomeration during methane reactions at 950 K for > 10 h. The Brönsted acid sites formed during carburization and oligomerization of MoC x species ultimately become covered with hydrogen-deficient reaction intermediates (H/C ∼ 0.2) or unreactive deposits. The highly dispersed nature of the MoC x clusters was confirmed by detailed simulations of the XAS radial structure function and by the low temperatures required for the complete oxidation of these MoC x species compared with bulk Mo 2 C. Initial CH 4 reactions with MoO x precursors are stoichiometric and lead first to the removal of oxygen as CO, CO 2, and H 2 O and to the introduction of carbidic carbons into the reduced structures. As carbidic carbon passivates the surface, C−H bond activation reactions become catalytic by the coupling of this activation step with the removal of the resulting CH x species to form C 2 hydrocarbons, which desorb to re-form the MoC x sites required for C−H bond activation steps.